1. Field of the Invention
The present invention relates to a micellar slug suitable for use in a micellar drive for recovering oil (i.e., petroleum) from subterranean reservoirs. More specifically, it relates to a micellar slug having large intake amounts of oil and brine in the resultant micro-emulsions and a good dilution stability of the micro-emulsion, capable of recovering oil from subterranean reservoirs having a high salt content at a high oil recovery efficiency.
2. Description of the Prior Art
Micellar drive is known in the art as one of the so-called "enhanced oil recovery (EOR)" methods for recovering oil from oil-bearing subterranean reservoirs. According to the known micellar drive methods, microemulsions comprising water, oil, surfactants, and cosurfactants are injected under pressure into the subterranean reservoirs, or aqueous solutions of water, surfactants, and cosurfactants are injected into the subterranean reservoirs to form micro-emulsions therein, for the recovery of oil remaining in the subterranean reservoirs. Various processes and chemicals have been heretofore studied in the art. The micellar drive methods and chemicals are disclosed in, for example, U.S. Pat. Nos. 3,506,070, 3,990,515, 4,017,405, 4,018,278, 4,059,154, and 4,066,124. These prior arts disclose various kinds of anionic, cationic, and nonionic surfactants as surfactants suitable for use in the micellar drive methods and also disclose various kinds of lower alcohols as cosurfactants suitable for use in the micellar drive methods.
However, the micellar drive methods utilizing surfactants and lower alcohols are disadvantageous in that, since the partition coefficients of the surfactants and the lower alcohols to oil and water are different, the desired high oil recovery capability of the microemulsions cannot be maintained in the subterranean reservoirs due to the dilution thereof with oil and water, and the formation of the micro-emulsions having large intake amounts of oil and water is difficult.